Introduction Anisotropic etching :In semiconductor technology ,isotropic etching isnon-directional removal of material from asubstrate via a chemical process usingan etchant substance. The etchant may be acorrosive liquid or a chemically active ionized gas,known as a plasma. Anisotropic chemical wet etching is one of the keytechnologies for fabricating microstructures on asingle-crystal silicon wafer
Experimental method Purpose : compare the anisotropic etchingproperties of KOH and TMAH(Tetramethylammonium hydroxide) Solutions : KOH : strong bases TMAH : quaternary ammonium hydroxide (QAH)solutions
Specimens Using hemisphericalspecimens of single crystalsilicon whose surfaceexhibited everycrystalographic orientation ,in order to evaluate theetching properties as afunction of the orientation. The orientation dependencein the etching rates of thesurface crystals significantlydiffered between twoetchantssolid hemispherical specimen of single-crystal silicon :- R= 22 nm- sphericity less than 10 µm- surface roughness : 0.005-0.007 µm
Measuring the profile All crystallographic orientations appeared on thehemispherical surface Measuring the profile before and after etching and notingthe change enabled us to calculate the etching rate for anyorientation. The profile was measured using a UPMC550-CARAT(Carl Zeiss , made in Germany ) 3-D measuring machine. The surface profile was probed every 2 oof latitude rangingfrom 20 oto 90 oand every 2 oof longitude ranging from 00to 3600. The total number of probe points was 6480
Etching conditions Etching bath(made of Teflon) was immersed in awater bath that had heater elements embedded inthe walls and bottom. The silicon specimen was first heated to the etchingtemperature in a dry container( immersed in thewater bath) Transfer the specimen into the etching bathcontaining 1 liter of etchant. The specimen was held in a Teflon basket so that itshemispherical surface was at least 5mm away fromthe interior of the basket
Etching conditions Control the etching conditions: A silicon chip was thrown into the etchant prior to theexperiments to estimate the time necessary to etch thehemisphere Fresh etchant was used in every subsequent experiment A magnetic stirrer was used to equalize the temperaturein the water bath The temperature distribution in the etching bath was veryuniform (no variation being detected in the verticaldirection along the center axis of the etching bath) The stability of the temperature : ± 0.9 0C
Comparison of the morphologies at four orientations of the surface etchedin KOH and TMAH solution
Variation in the range of themeasured etching ratesVariation range inetching rates amongfour equivalent (110 )planes at the samelatitude of 300on thesame hemispherenormalized by that ofthe (110 ) plane at thetop
Variation in the range ofthe measured etching ratesThe effects of etchant circulation can beignored in the case of KOH solutions, but not inthe case of TMAH solutions, especially whenthe concentration is 10 wt.%. In the case of20wt.% TMAH, however, the effects of etchantcirculation were negligible
conclusions The orientation dependence was quite different for the ( 111 ) and ( 221)planes in TMAH and KOH solutions. This suggests that there aredifferences in the etching mechanisms of the two etchants in terms ofcrystallographic orientation. The etching rate ratio of (111)/(100) in TMAHwas about twice that in KOH . The etching rates depended on the concentration of KOH and TMAHsolutions. Many orientation planes had a peak in the etching rates as afunction of the concentration of the solutions. The peak in KOH was at 25wt.%. The peak in TMAH was at 20 wt.%. The activation energies in KOH and TMAH were almost the same for the(100) , (110) , and (320) planes but not for the (221) and (111) planes.The activation energies for the (110) and (111) planes at KOHconcentration of 34 wt.% were 0.60 and 0.50 eV, respectively. The effects of etchant circulation can be ignored for KOH, but not forTMAH solutions. The roughness of etched (100) surface, which is the smoothest of all theorientations, was 0.01 m m with KOH and 0.4 m m with TMAH solutions.The roughness in-creased as the etchant concentration decreased.
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